The relationship between porosity and thermal conductivity of concrete

This thesis reports the details and results of an experimental study into the effect of porosity on the thermal conductivity of lightweight and normal-weight concretes. The concretes were made from either Pellite, limestone or quartzitic coarse aggregates, with or without air-entraining agent. Their density varied from 1550 to 2350 kg/m3 and their porosity from 10 to 39 per cent. The thermal conductivity of all the concretes was measured in accordance with B.S. 874: 1988, using a plain hot-plate technique and the total porosity and pore size distribution of each mix were determined using vacuum saturation and Mercury Intrusion Porosimetry. Micromorphological changes were additionally assesed by: a) using thermal analysis methods to monitor lime and non-evaporable water content of mortars and also to monitor the carbonation process by quantitative evaluation on mortar samples. b) assessment of the depth of carbonation on concrete cubes. c) using Scanning Electron Microscopy on coarse aggregates and mortar samples. For characterization purposes, the experimental programme also involved studying the properties of fresh concrete, such as workability and air content and other engineering properties of hardened concretes, such as compressive strength. The results obtained from the individual tests were statistically analysed using a standard `Statistical Analysis System' (SAS) package. This enabled detailed examination of the relationship between the different properties to be assessed. Resulting from this, a model was developed which enabled the thermal cocductivity of a concrete to be estimated knowing its dry density, total porosity and median pore diameter. The findings of this investigation confirm that in general, the thermal conductivity of concretes are directly related to density and inversely related to porosity and pore diameter and that density and total porosity have the highest coefficient of correlation respectively.